The present invention relates, generally, to a pump source with increased pump power for optical broadband Raman amplification of a wavelength-division multiplexed (WDM) signal.
Booster amplifiers are required along the optical transmission path for long-distance transmission of a WDM signal. For broadband amplification, Raman amplifiers with a number of pump lasers must have a high gain and a flat gain spectrum for the channels of the WDM signal in L band and C band. Pump sources with pump lasers having different output wavelengths are required for this purpose. The wavelengths and the output powers of the pump lasers must be selected optimally.
Such a Raman amplifier is described, for example, in a prior German patent application with the file reference P 10048460.3, in which pump signals which are produced by a number of pump lasers and are at different pump wavelengths are fed in opposite directions into the transmission fiber so that a WDM signal is amplified by stimulated Raman scatter. The powers of the pump lasers are chosen, and the pump wavelengths are arranged, such that all the channels of the WDM signal have a flat profile of the signal-to-noise ratio (OSNR) at the transmission fiber end. In addition, a filter (F) is connected to the output of the Raman amplifier in order to equalize all the channel levels, in order for the level profile of all the channels likewise to be flat. In consequence, all the channels of a WDM signal have the same signal-to-noise ratio and are at the same level at the fiber end.
Pump laser output powers of several hundred milliwatts are required for the gain values of approximately 10 dB to 20 dB that are relevant in practice. Until now, two major technologies have been known for producing such high pump powers. The first technology is based on a high-power Raman cascade laser, which has one row of low-cost laser diodes with output wavelengths around 920 nm, and has a fiber laser doped with an element from the group of rare earths. The output signal from the laser diode row is fed into the outer core, which carries multiple modes, of the active fiber of the ytterbium doped or neodym-doped fiber laser. This produces a transverse monomode output signal in the wavelength range around approximately 1100 nm or 1064 nm. In order to convert the signal in the wavelength region around approximately 1450 nm which is desired for pumping the Raman amplifier, the signal is also converted in a number of stages in a Raman cascade laser with fiber gratings and an efficient amplifier fiber. This allows output powers of well above 1 watt to be produced. The output signal from the Raman cascade laser has a narrow spectral width, however, and thus is not suitable on its own as a pump for a broadband Raman amplifier.
The second technology uses a so-called monomode WDM pump source with laser diodes which emit at different wavelengths (from about 1400 nm to 1500 nm) and which emit a transverse monomode output signal. A fiber grating is provided in the output fiber of the individual laser diodes for stabilization at the respectively desired pump wavelength. A wavelength-selective WDM multiplexer then combines all the pump signals. The pump wavelengths and the pump powers of the individual laser diodes are chosen such that the Raman amplifier has a flat input/output gain spectrum. This technology has the disadvantage that very high-power, and hence expensive, laser diodes must be used, since high powers are required for the individual pump wavelengths. In some cases, it even may be necessary to use two laser diodes rather than one laser diode, whose pump signals at identical wavelengths are combined via a polarization multiplexer. Since a number of very high-power and relatively expensive transverse monomode laser diodes are required, this technology is associated with high costs.
An object of the present invention is, thus, to specify a pump source with a high broadband output power for Raman amplification of a WDM signal, in which there is no need to use expensive laser diodes.
The fundamental idea of the pump source designed according to the present invention is the combination of at least one high-power laser, particularly a Raman cascade laser, with a number of monomode laser diodes with a low output power. This has the advantageous feature that low-power, and hence low-cost, pump lasers are used for the monomode laser diodes, and very high-power transverse monomode, and therefore also low-cost, lasers are used for the Raman cascade laser.
By transmitting the output signal from the cascade laser and all the output signals from the monomode laser diodes via a conversion fiber, the pump signals from the monomode laser diodes are subjected to Raman amplification. They then can be used as pump signals for Raman amplification of the WDM signals to be transmitted. This efficiently increases the originally low output powers of the monomode laser diodes.
A number of arrangements of the pump source according to the present invention are possible, which require only low output powers from the monomode laser diodes and can, nevertheless, produce the high pump powers which are required for a flat gain spectrum in the wavelength range of the WDM signals.
A coupling device for combining the pump signals at the various pump wavelengths has a greater number of inputs than the number of pump signals, so that the desired increases in the output powers for the monomode laser diodes are achieved during Raman amplification of the pump signals both for small and large wavelength separations between the pump signals from the cascade laser and the monomode laser diodes.
The conversion fiber of the pump source according to the present invention may have a number of series-connected conversion fiber sections. The pump signals from the monomode laser diodes are fed into different conversion fiber sections, depending on the wavelength separations between the pump signal of the cascade laser and the further pump signals to be amplified. An increase in the low Raman amplification for the pump signals at short wavelengths is achieved during transmission via a large number of conversion fiber sections. Individual monomode laser diodes or various modules of monomode laser diodes also can be coupled in between different conversion fiber sections.
In order to increase the output power amplification of monomode laser diodes with short wavelengths, low-power monomode laser diodes are used whose output power decreases as the wavelengths increase. This takes account of the increase in the output power by Raman amplification, which increases as the wavelength separations increase between the pump signals of the Raman cascade laser and those of the monomode lasers.
Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures.